Synthesis gas comprising hydrogen and carbon oxides is typically produced by steam reforming and/or partial oxidation of hydrocarbon feedstocks.
WO03/051771 describes a process for preventing unwanted side reactions that occur between carbon monoxide present in a heat exchange medium, including a secondary reformed gas, and the catalytically active metals present on the exterior (shell-side) surface of the steam reformer tubes by adding a passivating compound to the heat exchange medium before it enters the shell side of the heat exchange reformer. The temperature in the shell side of a heat exchange reformer is typically well above 600° C. and this is sufficient to decompose the passivation compound on the metal surface thereby forming stable species on the tube surfaces that are resistant to attack by carbon monoxide. Unlike the downstream apparatus, the internals of the heat exchange reformer apparatus, which are in contact with the reformed gas on the shell-side, are typically fabricated from high temperature resistant low-iron, Ni—Cr alloys and it is believed that the Ni is the source of unwanted side reactions. Whereas phosphorus compounds amongst other compounds are described as suitable passivating compounds for the shell side of the heat exchange reformer there is no suggestion that they may be effective at lower temperatures in low-alloy steel apparatus, such as low-alloy steel apparatus, downstream of the reformer itself. Indeed in WO03/051771 it is suggested that where the passivating species is volatile that absorbent beds be incorporated downstream of the reformer apparatus to recover the passivating species.
In WO 00/09441 a reforming process is disclosed wherein corrosion of the shell side of a heat exchange reformer by a high temperature secondary reformed gas used as heat exchange medium may be reduced by introducing a sulphur compound, such as dimethylsulphide into the secondary reformed gas after it leaves the secondary reforming apparatus and before it enters the heat exchange reformer as heat exchange medium. The amount of sulphur compound necessary to obviate such corrosion problems was stated to be such as to give a sulphur content of 0.2-20 ppm by volume in the secondary reformed gas. Because sulphur only binds weakly to the catalytic metal sites, to prevent contamination and deactivation of catalysts in subsequent process steps, the sulphur compounds were removed by passing the secondary reformed gas exiting the heat exchange reformer through a bed of a suitable absorbent for sulphur compounds, such as zinc oxide. However, the provision of sulphur-removing apparatus adds additional cost and complexity to the reforming process.
WO 01/66806 describes a method for preventing nitridation and/or carburization of metal surface by adding a sulphur compound and a lower amount of a phosphorus compound to the process gas in contact with the metal. It is suggested that the phosphorus compound acts to prevent corrosive sulfidation of the metal surfaces that occurs when sulphur compounds are added to prevent the nitridation and/or carburization reactions. In the examples, addition of 2 ppm of phosphorus in the form of phosphorus pentoxide to a gas containing 20 ppm of hydrogen sulphide completely prevented the sulphur compound from adhering to 12% chromium steel surfaces. However, it was also shown that if the metal surface was subjected to phosphine pre-treatment, nitridation of the metal was not prevented. The requirement for sulphur as well as phosphorus leads to the requirement for expensive sulphur recovery before the syngas may be used in subsequent catalytic processes. Furthermore the unwanted nitridation side reaction was not prevented by the pre-treatment using phosphine indicating that it was ineffective at passivating the metal surface.
We have found a method that overcomes the problems of the above processes.